So you've seen a Great Ball Contraption at a LEGO display or on the Internet, and you want to make some modules of your own? Great! Here is some information to help you get started. If, after reading this, you have any questions, check the FAQ or post your question in the forums.

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As you consider building GBC modules there are some skills/knowledge that will ease your experience significantly. If you don't have these skills now don't worry, but pay careful attention to how these techniques and principles are exploited in GBC modules.

Building with LEGO: You should have an understanding of how LEGO elements, in particular Technic elements, connect with each other.

Advanced building with LEGO: You should have an understanding of basic SNOT techniques, and how to brace structures so that they are sturdy.

Basic mechanics: You should have an understanding of force, torque, how levers and gears work, etc.

Patience/perseverance: Perhaps most important. Building a good GBC module is not an easy thing to do. But then, neither is anything worth doing! Be prepared to redo, several times if need be, most parts of your module, and don't expect to be able to complete a whole module in one sitting.

Since modules are usually displayed together, it's important to follow the building standard so that your module will be compatible with the others. Familiarize yourself with it and pay the most attention to basket dimensions and throughput rate, because deviations from those rules are the most difficult to compensate for when connecting it to other modules.

To properly test your module, you will need some Lego balls. This is an investment that all GBC builders have to make, but some LUGs may allow you to borrow a few. For testing individual parts of the module, having 5-10 handy will be useful. To properly test your module though you'll need at least 30 balls, which is the maximum number that can be received in a batch. If the mechanism requires priming (e.g. Ball pump), additional balls will be needed.

If obtaining balls is difficult and you only want to run the contraption at home, you can of course use anything for balls - marbles, large jewelry beads, even certain candies will suffice! It's likely to end up incompatible with the standard Lego balls though, as properties such as size and bounciness will differ. One possibility is making your modules compatible with both types of balls. You can then use your large collection of non-standard balls when playing at home and keep a smaller collection of standard balls for testing.

Your first module can be your own design, or it can be a copy of someone else's, but it should be simple. Since 2014 the Brickworld event has run a workshop specifically for first-time GBC builders. The modules built are designed to be simple, reliable, and constructed of relatively few, relatively common parts. Instructions for the modules for the first two workshops can be found here and here.

A beginner's GBC module will typically have three components: an input area, some form of lift mechanism, and a way to safely deliver balls to the following module. More advanced modules may also have some kind of widget that does something interesting with the balls before moving them on (in fact, some modules have the widget as the whole focus - the lifter is just there to get the balls high enough!), but we won't worry about that for now.

Most GBC modules are built from the inside out. That is, the core of the module, the lifting mechanism, is designed first, and the input and output parts are built around it. Once you've got your lifting mechanism built, and you're comfortable that it doesn't spill or break down (it will probably take several major revisions to get to this point!), the next thing to build is usually the input area. Depending on your lifting mechanism, this could be as simple as an appropriately-sized basket with one side open to the base of your lifter, or you might need some complicated ramp arrangement to get the balls in the right position to be loaded onto your lifter. One thing to beware of is that any place where the ball path narrows significantly is likely to get blocked (typically by two balls trying to go through at once) unless there is something providing some constant agitation to break up the clots. It will probably take several more revisions until your input area is feeding your lifting mechanism reliably.

All that's left to build now is the output section. It needs to unload balls from the lifter without jamming, get the balls clear of your mechanism and any supporting structure (including baseplate), and deliver them politely to the next module. This is probably the easiest part of the module, but it can be surprisingly nontrivial to accomplish.

Once you've got a complete path from input to output, then the serious testing can begin. See how it runs with a constant stream of balls one at a time. See how it runs with a whole bunch of balls dumped in all at once. See how it runs with no balls at all. Get another module (from a friend, or build a second copy, or build a different module altogether!) and join them together, in both orders, to see how the interface between the two modules works.

Now that you've got one (or more!) modules that are fairly reliable and don't leak too many balls. It's time to display them alongside other modules at a public event. Here are some tips to increase your chance of being asked back again:

Make sure the coordinator knows beforehand what you're intending to bring. It helps them plan the display, and they can let you know of any requirements specific to that event.

Make yourself available to help set up and tear down the display outside of just looking after your own modules.

For at least the first couple of hours, be nearby so that you can fix any problems with your module.

GBCs require constant tending, so offer to watch for a while so that others can have a break.

In other words, don't just rock up, dump your module on the table and say "Here's my module, I'll be back at the end of the event to pick it up kthxbye".

Chances are this is the first time your module will be run for more than an hour or so at a stretch, so be prepared to find a bunch more problems. Maybe you've got a connection that works itself loose over the course of a day. Maybe you've got an axle that's bearing too much weight as it spins - the module might work fine, but the build-up of ABS dust is noticeable. Whatever it happens to be, you're likely to take your module home with some form of improvement to make. This is not a bad thing - it means that next time it'll work that much better.

Once you've got a couple of simple modules under your belt, you should then have the confidence to try something a bit more complicated, and maybe include a widget or two. Hopefully you've now got an idea of the sorts of traps and pitfalls you will encounter, and you can build to avoid them. Welcome to the ranks of GBC builders!